Published: October 31, 2018

Rice, M. L., Zubrick, S. R., Taylor, C. L., Hoffman, L., & Gayán, J. (2018). Longitudinal study of language and speech of twins at 4 and 6 years: Twinning effects decrease, zygosity effects disappear, and heritability increases. Journal of Speech, Language, and Hearing Research, 61(1), 79-93.


Mabel L. Rice, Stephen R. Zubrick, Catherine L. Taylor, Lesa Hoffman, and Javier Gayán.



This study investigates the heritability of language, speech, and nonverbal cognitive development of twins at 4 and 6 years of age. Possible confounding effects of twinning and zygosity, evident at 2 years, were investigated among other possible predictors of outcomes.


The population-based twin sample included 627 twin pairs and 1 twin without a co-twin (197 monozygotic and 431 dizygotic), 610 boys and 645 girls, 1,255 children in total. Nine phenotypes from the same comprehensive direct behavioral assessment protocol were investigated at 4 and 6 years of age. Twinning effects were estimated for each phenotype at each age using general linear mixed models using maximum likelihood.


Twinning effects decreased from 4 to 6 years; zygosity effects disappeared by 6 years. Heritability increased from 4 to 6 years across all 9 phenotypes, and the heritability estimates were higher than reported previously, in the range of .44–.92 at 6 years. The highest estimate, .92, was for the clinical grammar marker.


Across multiple dimensions of speech, language, and nonverbal cognition, heritability estimates are robust. A finiteness marker of grammar shows the highest inherited influences in this early period of children’s language acquisition.